Stay at the Sheraton Kauai. There is a better chance to get upgraded and there is also the cash and points option. You are spending 46k points and staying 4 nights. I think that's a waste since you get the 5th night free. Have you tried the cash and points option it's 4k points and $60 and then try to upgrade with points?

Since your husband is a plat I don't expect there to be a problem with getting a upgrade. Tourism is slow in Hawaii now.